Most commercial color printing uses three or four subtractive primary colour inks, typically: Cyan (C), Magenta (M), Yellow (Y) and usually also Black (K). A color image to be printed is first separated into a respective primary-color image (color separation) for each ink and these images are then printed as superimposed layers (color planes), using a halftoning process.
In order to keep consistent colors within a page and among pages, printing devices need to maintain consistent registration among the color planes. Registration changes can also lead to the appearance of moiré and banding artefacts. Registration changes may arise for a variety of reasons such as mechanical, optical and electrical features of the printer, mechanical shocks, deformations of the printing substrate (typically paper), etc.
Various techniques are known for monitoring color plane registration (CPR) the most common of which is to print a CPR test page with both nominally-aligned horizontal lines and nominally-aligned vertical lines in each of the inks being used (as used herein in relation to color plane registration, the term “vertical” means the direction of process flow through the printing system whereas “horizontal” means the direction transverse to the process flow). The printed test page is then imaged to form a digital image from which the out-of-alignment distances between the different-colored lines are measured. Typically, the lines of one color (usually black) are used as a reference and the distance of each of the other lines from the black is then measured to give mis-registration values in both the vertical and horizontal directions.
CPR test pages are used, for example, during initial set up of a printing system; they are also used in diagnostic processes where, following detection of imperfections in printed jobs, various different tests are carried out using test-specific test pages, to identify the printing system fault giving rise to the detected imperfections.